My invention relates to an improved method of cutting or slicing wafers from ingots of semiconductor materials. In particular, my method involves the addition of hollow microspheres to the adhesive that bonds said ingot to a mounting beam during the slicing operation.
The preparation of substrates, usually silicon, for the fabrication of semiconductor and photovoltaic devices requires a number of precisely controlled chemical and mechanical steps. The substrate material is first prepared in a very pure state by whatever preparation and refining methods are required. This material is then crystallized to provide a very large single crystal in the form of a cylindrical ingot. These ingots are then sawed or sliced into wafers that are usually lapped and polished to provide a flat surface for the production of sophisticated electronic coponents.
Slicing the ingots into wafers is a very important step in the process, since the wafers must be of uniform thickness, have a flat profile and be free of stress produced by slicing. One of the factors that is required to achieve these requirements is that the ingot be held very securely during the slicing operation. The method currently used involves bonding the ingot to a cutting or mounting beam, which is usually graphite, with an epoxy resin or wax. The graphite cutting beam is coated with the epoxy, and the silicon ingot is placed on the beam. The epoxy cures for 12 to 16 hours before a diamond saw or wire saw is used to slice the ingot into wafers. The saw cuts the wafers, the adhesive and part of the mounting beam. The wafers are removed from the cutting beam by breaking the epoxy adhesive bond by mechanical or chemical means.
Many of the problems associated with slicing the ingot are attributable to, or complicated by, the epoxy adhesive. The long cure time required to minimize heat induced stress is an obvious problem. The epoxy can be cured much faster, but with the penalty of a higher exotherm and more heat-induced stress. During cutting, the inside diameter saw blade or wire saw penetrates the adhesive and part of the mounting beam as well as the silicon ingot. Debris from these cuts, especially from the epoxy, clogs the diamond-nickel matrix of the saw blade, and periodically an operator must dress the blade manually with an abrasive dressing stick to prevent damage to the wafers. The problems add time and cost to the process. In addition, removal of the sliced wafers from the cutting beam is accomplished by breaking the wafer/ingot bond. Chipping and rejects are common with epoxies. A very small error in the proportions of the epoxy components can result in a very large number of rejects.
The wafer fabricating industry would consider an additive that addresses one of these problems a great improvement. A single additive to the system that prevents or ameliorates thermally induced stress and the clogging problem as well as reducing rejects provides a profound change in the system.
It is an object of this invention to provide an improved epoxy system that includes hollow microspheres, allowing faster, stress-free cure times, cleaner slicing and improved separation of the bonded part from the support beam when used for bonding ingots to a cutting beam.